Intramolecular Halogen Bonding in Solution: 15N, 13C, and 19F NMR Studies of Temperature and Solvent Effects

A model system for the investigation of intramolecular halogen bonds is introduced. Two molecules capable of intramolecular halogen bonding have been studied in comparison with eight control compounds by N-15, C-13, and F-19 NMR spectroscopy. Iodine- and bromine-centered halogen bonds are indicated by decreases in the N-15 NMR chemical shifts of the halogen bond acceptor atom of approximately 6 and 1 ppm, respectively. C-13 NMR chemical shifts of the alkynyl carbons in 2-ethynylpyridine systems are good indicators of halogen bonding, with differences of up to 2.4 ppm between halogen-bonded and related control compounds. Halogen bond strengths in different solvents, as indicated by F-19 NMR chemical shifts, decrease in the following order: Cyclohexane > toluene > benzene > dichloromethane > acetone > pyridine. Chemical shift effects associated with the structural and electronic properties of intramolecular halogen-bonded systems are modeled well by calculations at the B3LYP/6-311+G(2d,p) level of theory.